1. Field Of The Invention
The field of art to which this invention pertains is the solid bed adsorptive separation of halogen substituted aromatic hydrocarbons. More specifically, the invention relates to a process for separating para- and ortho-chlorotoluenes by adsorption chromatography with L-type zeolites.
2. Background Information
A wide variety of halogenating agents, catalysts and reaction conditions are used in processes for halogenating toluene. For example, mixtures of monochlorotoluenes are obtained by chlorination with certain Lewis acid halide catalysts, including the chlorides of aluminum, tin, titanium and zirconium, and with ferric chloride as catalyst. Other methods of obtaining monochlorotoluene isomers include the non-catalytic nuclear chlorination of toluene in various solvents, and the use of hydrogen chloride as a chlorinating agent in both liquid and vapor phase systems. In some cases, the mixture of monochlorotoluenes may contain more than 70% of the orthoisomer. However, considerable para-isomer content is also obtained, which in certain applications is detrimental to further utilization of the ortho-chlorotoluene product. When preparing monochlorotoluene isomers by direct chlorination routes, it has been found that less than 1% of the isomers produced will comprise the meta isomer, except that the meta isomer may comprise up to about 2% with ferric chloride. Consequently, feedstocks for this invention will often contain a small proportion of the meta isomer which is not removed prior to the separation of the para- and orthoisomers.
Chlorotoluenes are important as chemical intermediates. Both mono-and dichlorotoluenes are used in the manufacture of pesticides, dye stuffs, pharmaceuticals and peroxides. Halogen substituted toluenes are also employed as solvents.
One application for highly pure ortho-chlorotoluene (o-CT) is in the further chlorination of o-CT by the same routes mentioned above to obtain a mixture of dichlorotoluene (DCT) isomers, i.e., 2,3-, 2,4-, 2,5- and 2,6-DCT, from which 2,5-DCT can be separated chromatographically with alkali metal-exchanged L zeolites as disclosed in McCulloch et al U.S. Pat. No. 4,922,040 However, if the o-CT feed is contaminated with para-chlorotoluene (p-CT), 3,4-DCT is additionally formed. 3,4-DCT and any unreacted p-CT cannot be easily separated from the 2,5-DCT extract product by either simple fractionation or by the chromatographic separation referred to in said application since 3,4-DCT and p-CT are also adsorbed by the L zeolite. Hence, the purity of the 2,5-DCT product of the latter separation process is lowered to the extent that p-CT contaminates the o-CT being chlorinated.
Separation of ortho-chlorotoluene (bp 157.degree.-59.degree. C.) and para-chlorotoluene (bp 162.degree. C.) is difficult due to the close boiling point range of these isomers. In order to accomplish a separation of these isomers by fractionation, it is necessary to use a high efficiency isomer separation column. This invention simplifies the separation procedure by providing a more effective adsorptive separation method.
Japanese Patent Application Nos. 11,884/82 and 50,440/83 deal with the separation of meta-chlorotoluene (m-CT) from a mixture of chlorotoluene isomers. In the first application, m-CT is adsorbed on a Y zeolite containing silver and potassium cations. The second application teaches the separation of m-CT from o-CT and p-CT on a Y zeolite adsorbent containing sodium and copper cations as essential components.
The separation of halogenated aromatic isomers using X zeolites containing alkali metal or alkaline earth metal cations is disclosed and exemplified in Fleck et al U.S. Pat. No. 2,958,708 and Japanese Patent Publication 5155/60. Japanese No. 5155/60 further discloses the use of chlorobenzene as desorbent.
U.S. Pat. No. 4,605,799 discloses the adsorptive separation of halogenated toluenes, e.g., a mixture of chlorotoluene isomers with a Y-type zeolite exchanged with sodium or potassium cations and a desorbent such as 3,4-dichlorotoluene.
I have found an adsorbent, which, in combination with certain desorbent liquids, will selectively adsorb all the chlorotoluene isomers, except o-CT, which is relatively non-adsorbed and which elutes near the void. Thus, the largest component of the feed, o-CT is eluted as raffinate and the minor components are adsorbed and eluted as extract by desorption with the desorbent. This so-called rejective separation of the major components is desirable since utilities are lower and adsorbent capacity requirement for the adsorbed components is lower per unit of output product. Another advantage of the present invention is that the separation can be accomplished with the same adsorbent/desorbent combination used for the separation of the dichlorotoluene chlorination products of o-CT, i.e., as disclosed in the aforementioned U.S. Pat. No. 4,922,040.
Methods for forming the crystalline powders into agglomerates are also known and include the addition of an inorganic binder, generally a clay comprising a silicon dioxide and aluminum oxide, to a high purity zeolite powder in wet mixture. The blended clay zeolite mixture is extruded into cylindrical type pellets or formed into beads which are subsequently calcined in order to convert the clay to an amorphous binder of considerable mechanical strength. Clays of the kaolin type, water permeable organic polymers and silica may also be used as binders.
The invention herein can be practiced in fixed or moving adsorbent bed systems, but the preferred system for this separation is a countercurrent simulated moving bed system, such as described in Broughton U.S. Pat. No. 2,985,589, incorporated herein by reference. Cyclic advancement of the input and output streams can be accomplished by a manifolding system, which are also known, e.g., by rotary disc valves shown in U.S. Pat. Nos. 3,040,777 and 3,422,848. Equipment utilizing these principles are familiar, in sizes ranging from pilot plant scale (deRossett U.S. Patent 3,706,812) to commercial scale in flow rates from a few cc per hour to many thousands of gallons per hour.
The functions and properties of adsorbents and desorbents in the chromatographic separation of liquid components are well-known, but for reference thereto, Zinnen et al U.S. Pat. No. 4,642,397 is incorporated herein.
It has now been discovered that L-type zeolites having potassium cations or a mixture of sodium and potassium cations at cation exchange sites are suitable adsorbents for the separation of ortho-chlorotoluene (o-CT) from other isomers of chlorotoluene.